United States patent application Ser. No. 256,864 discloses an actuator which is powered by gaseous propellant flowing from an evaporator thermally coupled to a thrust engine to an ullage of a storage tank storing liquid propellant in a portion of the storage tank. The actuator disclosed in Ser. No. 256,864 does not dynamically control the mass flow of gaseous propellant to the ullage as a function of power being outputted by a power takeoff driven by the gaseous propellant flowing to the ullage. A parallel gas path bypasses a minimum mass flow of gas past the power takeoff to provide a mass flow necessary to maintain the ullage at a desired pressure. An expansion valve contained in the parallel gas path does not dynamically vary the flow of gas in the path as a function of power outputted from the power takeoff. Gas discharged from the power takeoff is added to the minimum mass flow in the parallel gas path which is necessary to maintain the ullage. As a result, an overpressure condition can be created within the ullage which must be relieved by an overpressure valve which vents excess gas overboard. Therefore, a maximum power output from the gaseous propellant flowing to the ullage is not obtained as a consequence of a mass greater than that necessary to maintain the ullage flowing to the ullage necessitating venting overboard from the ullage.
Turbo actuators which are powered by a gas generator for driving a load in propulsion systems having first and second turbines which rotate in opposite directions to drive a common shaft are known. These systems utilize a control valve which divides a gas supply from a solid propellant gas generator between wheels of the turbine by directing the gas flow from one turbine to the other at a fixed frequency with pulse duration modulation. See "Turbo-Actuators Turn Flow Into Mechanical Output" by P. H. Stahlhuth, Hydraulics & Pneumatics, April 1974, pp. 89-92.